1. Field of the Invention
The present invention relates generally thermosetting resin compositions that include varying amounts of one or more thermoplastic resins, which provide elastomeric toughening of the resin. More particularly, the present invention involves compositions and methods wherein the amount of thermoplastic resin in uncured thermosetting resin compositions is maximized to provide desired levels of resin toughening. These relatively high levels of thermoplastic loading are achieved without increasing the viscosity and/or tack of the uncured composition to levels that would render them unsuitable for handling and processing.
2. Description of Related Art
Thermosetting resins are popular materials that are used in the fabrication of a wide variety of composite materials. Epoxy resins, bismaleimide resins and cyanate ester resins are examples of popular thermosetting resins. Composite materials typically include one or more resins that are combined with fibers or other support materials to form relatively lightweight structures that are relatively strong. In addition to their wide spread use as a matrix material in composites, thermosetting resins have also been used to provide a surface layer or finish coating on the exterior surface of composite materials. Composite materials and surface coatings or layers that utilize thermosetting resins have been widely used in the aerospace industry where their combination of lightweight and structural strength is particularly desirable.
In many situations it is desirable to modify thermosetting resins by adding one or more thermoplastic resins to the thermosetting resin mix. The use of thermoplastic resins provides an additional degree of flexibility or elasticity to the thermosetting resin which increases the toughness of the final cured resin product. Such thermoplastic toughened resins have been used both as resin matrix materials in combination with fibers and as coatings where a particularly tough resin is desired. Typical thermoplastic resins that have been used as toughening agents include polyethersulfone, polyetherimide and certain types of nylon and other thermoplastic polymers that can be dissolved in the thermosetting resin prior to ultimate cross-linking and cure.
In general, the degree of elastomeric toughening provided by the thermoplastic resin is related directly to the amount of thermoplastic resin that is incorporated into the thermosetting resin mixture. Accordingly, it is desirable in many situations to add as much thermoplastic resin as possible to achieve maximum toughening of the final cured resin. High thermoplastic loading is particularly desirable for thermosetting resins that are used as surface coatings on aircraft structures where extremely smooth and strong surfaces are desired in order to enhance the aerodynamics and appearance of the structure.
The amount of thermoplastic resins that can be added to thermosetting resin mixtures is limited by a number of practical considerations relating to processing of the uncured resin. For example, the viscosity of the uncured toughened resin must be kept low enough that the resin can be fabricated into thin films and/or used to impregnate or coat various materials. In addition, the tack of the resin must also be kept within certain limits that vary depending upon the particular fabrication process. For example, the resin cannot be fabricated into films or handled during fabrication of multi-component structures if the uncured resin is too tacky. High viscosity and high tack is a particular problem when uncured toughened resins are used in the fabrication of surface coatings. High viscosity and high tack cause handling problems that lead to surface imperfections that have a deleterious effect on appearance and may an adverse effect on aerodynamics. As a result, the amount of thermoplastic resin that is loaded into thermosetting resins for use in surface coatings is typically below 25 weight percent.
The thermosetting and thermoplastic resins that are used to make composite materials do not conduct electricity. The most common fibers used in composite materials (such as glass, ceramics and graphite) are not good conductors of electricity. This is a particular problem for aircraft because they operate in an environment where they may be exposed to lightning. It is well known that a lightning strike on an aircraft that includes substantial amounts of composite material may result in catastrophic failure of the aircraft. In order to protect against such catastrophic events, the composite material used in many aircraft are modified to include “lightning protection”. Lightning protection typically involves adding one or more layers of conductive material to the composite materials used to make the aircraft. The conductive layers are generally located on or near the exterior surfaces of the composite portions of the aircraft. The conductive materials used for lightning protection are usually thin films of metal (or other conductive material) or metallic mesh or fabric that is relatively lightweight.
There are many different ways to add lightning protection to the surface of a composite structure. A common procedure involves preparing a “prepreg” that is lightning protected and incorporating the prepreg into the surface of the composite structure. “Prepreg” is a term used in the composite industry to describe a composite precursor wherein one or more layers of fabric have been impregnated with uncured resin. The resulting pre-impregnated structure is typically stored for later use in fabricating the final cured composite structure. The preparation and use of prepregs is particularly desirable in the fabrication of aircraft and other critical structures because it allows the manufacturer to carefully control the amount of resin that is combined with a given amount of fabric. As a result, the final properties of the cured composite structure can be carefully controlled. Prepreg that is suitable for use as a lightning protection surface coating will typically include an electrically conductive layer located between a layer of surface finish resin and a supporting layer of fabric that has been pre-impregnated with resin. Examples of lightning protection prepreg and composites are set forth in the following U.S. Pat. Nos. 5,225,265; 5,470,413; 5,370,921 and 5,397,618.
Prepregs that are intended for use in providing lightning protection for composite structures should have certain desirable characteristics. The prepreg should be as light as possible to avoid adding unnecessary weight to the aircraft. The prepreg should be compatible with the underlying prepregs or other pre-cure composite materials used to make the final composite structure. The prepreg should also be relatively easily to handle and the surface finish provided by the cured prepreg should be relatively free of surface flaw or imperfections.